1. Field of the Invention
The present invention relates to a metal halide lamp and an automotive headlamp apparatus incorporating the same.
2. Description of the Related Art
U.S. Pat. No. 6,353,289 by Ishigami discloses a metal halide lamp suitable for an automotive headlamp that substantially contains no mercury (conveniently referred to as a mercury-free lamp, hereinafter). The inventor of this invention is one of the inventors of the present invention. The metal halide lamp has electrodes distant from each other by 5 mm or less and has, sealed in a hermetic vessel as a discharge medium, first and second halides and a xenon gas at 5 atmospheres or higher at 25° C. The first halide is a halide of a light-emitting metal, and the second halide is a halide of a metal for providing a lamp voltage. As the second halide, a halide of a metal that has a high vapor pressure and emits no or a relatively little visible light is used, besides a halide of a light emitting metal. If the mercury-free lamp described above is a small metal halide lamp used in an automotive headlamp, it is turned on with a lamp power of 60 W or lower. Still, the mercury-free lamp in a stable state is as luminous as mercury-containing lamps.
However, if the first and second halides are sealed in the mercury-free lamp in an inappropriate ratio, there arise problems of high color deviation, low lamp voltage and low luminous efficiency. The reason for this is as follows: in many cases, the second metal halide sealed in place of mercury for providing a lamp voltage is low in luminous efficiency and emits visible light which is low in chromaticity, and therefore, the second metal halide causes the luminous efficiency of the lamp to be reduced or the chromaticity thereof to be degraded or provides an insufficient lamp voltage depending on the amount thereof.
As described above, the lamp voltage depends on the amount of the second halide sealed. A larger amount of the second halide sealed can provide a higher lamp voltage. However, the quantity of visible light is reduced and the luminous efficiency of the lamp is reduced. On the other hand, a smaller amount of the second halide increases the luminous efficiency of the lamp. However, the lamp voltage tends to decrease. Furthermore, when the lamp power supplied to the metal halide lamp is fixed, a higher lamp voltage and a lower lamp current result in a low current capacity of a lighting circuit, and thus, can provide a relatively inexpensive lighting circuit.
In addition, the mercury-free lamp is inferior in rising of luminous flux, in general. In the case of an automotive headlamp, the xenon gas emits light just as with mercury immediately after activation. However, after that, evaporation of the halides is insufficient until the temperature thereof is increased to 400 to 600° C., which takes about 4 seconds after the activation. Thus, during this period, the xenon gas keeps emitting light. Accordingly, a lamp power more than twice as high as that in a stable state need be supplied to the lamp for about 4 seconds, and thus, a maximum lamp current need be supplied to the lamp for about 4 seconds after the activation. Nevertheless, if the first and second halides are sealed in an inappropriate ratio, the rising of luminous flux is delayed, and 80% or more of the total luminous flux cannot be attained within 4 seconds after the activation.
To the contrary, in the case of a conventional metal halide lamp which contains mercury as a medium for providing a lamp voltage (conveniently referred to as a mercury-containing lamp, hereinafter), the xenon gas first emits light immediately after activation, and then, mercury evaporates to emit light immediately and rapidly. Since the luminous efficiency of mercury is several times as high as that of xenon, the rising of luminous flux is relatively fast, and 80% of the rated luminous flux is attained 4 seconds after the activation. This luminous flux can be attained by supplying a power about twice as high as the rated lamp power immediately after the activation. The maximum lamp current flows only during a period immediately after the activation, decreases rapidly after a lapse of 1 to 2 seconds and becomes equal to or less than a half of the maximum after a lapse of 4 seconds.
However, the present inventors have found that, also in the mercury-free lamp, appropriate rising of luminous flux can be provided if the ratio between the first and second halides sealed is appropriately determined.
Furthermore, the mercury-free lamp cannot attain the same level of total luminous flux in a stable state as the mercury-containing lamp if the ratio between the sealed first and second halides is in appropriate. On the other hand, the mercury-containing lamp can easily attain a desired level of total luminous flux because the mercury vapor produces visible light by discharge, which largely contributes to the total luminous flux.